Contact Image Sensor (CIS) was first developed by Mitsubishi in the early 1980's as an alternative document scanning system to the conventional "lens reduction image sensing system" which utilizes a charge coupled device (CCD) or self-scanned photodiode array. The major advantages of the CIS scanning system over the conventional CCD imaging system are its compactness, light weight, low power consumption, and ease of system assembly.
FIG. 1 illustrates a conventional lens reduction image sensing system using a CCD array. An original document 1 is illuminated by a light source 2. Since a CCD image sensor 3 is typically approximately one inch long, an optical lens 4 is required to reduce the image of the text on the document 1 so that a full-width image can be received in the CCD image sensor 3. In addition, to obtain the necessary reduction, an optical distance of 10 to 30 cm is required between the CCD image sensor 3 and the document 1. This optical separation distance necessitates a rather bulky assembly for the overall scanning device, and for this reason, some prior art devices use sophisticated (hence expensive and difficult to manufacture) folded optical schemes to reduce the total physical size of the assembly.
FIG. 2 depicts a contact image sensor (CIS) system which is an improvement on the system shown in FIG. 1. In this device the optical reduction system is replaced with a full-width rod/lens system 5. This system allows one-to-one scanning of the document because the rod lens 5 and a hybrid image sensor 6 are of the same width as (or greater width than) the document to be scanned. This arrangement reduces the distance required between the image sensor and document being scanned to less than 2 cm.
A cross section of such an improved prior art imaging system utilizing a hybrid image sensor chip 6 is shown in FIG. 3, which depicts the arrangement of the components within a housing with a cover glass 7 to receive documents. FIG. 4 is a block diagram of such an imaging system, with FIG. 5 showing details of the construction of a prior art hybrid image sensor array 6. In this hybrid package, a plurality of individual sensor chips 61 are butted end-to-end on a single substrate. The number of individual sensor chips chosen is dependent upon the desired width of scanning. The hybrid sensor array 6 also contains signal-processing means to serially activate the individual sensor chips and to process the output signals.
A block diagram illustrating the function of a typical prior art individual sensor chip 61 is shown in FIG. 6, with details of the sensor elements shown in FIG. 7. The structure and function of this sensor chip is described in U.S. Pat. No. 5,299,013, issued on Mar. 29, 1994. With reference to FIGS. 5-7, the individual sensor chip 61 comprises an array of photodetectors, an array of multiplexing switches, a digital scanning shift register, built-in buffers, and a chip enable (chip selector). In operation, the hybrid sensor chip 6 is triggered by a start pulse to the first-in-sequence individual sensor chip 61 which serially activates the photodetectors on the first individual sensor chip 61. After the signal from the last photodetector element of the first individual sensor chip 61 is read, an end-of-scan (EOS) pulse is generated so that the next sensor chip in sequence is triggered.
The individual sensor chips 61 of most prior art devices utilize npn (or pnp) phototransistors as the sensing elements, as illustrated in the circuit diagram shown in FIG. 7. The npn phototransistors provide some current gain for the detected light signal, and thus serve to increase the photosensitivity of the device. However, phototransistors are subject to several inherent shortcomings. The phototransistor sensing array exhibits rather large photo-response non-linearity, thresholding problems at low light levels (waterfall effect), and substantial problems with image lag or carryover of portions of previous images to new scans (some times called residual image). Because of these problems, contact image sensor arrays using phototransistors as sensing elements are seldom used in scanner applications which require color or wide gray-scale linearity. Charge coupled devices (CCD) have thus generally been utilized in devices that are to be used for reproducing color or wide-range gray-scale images, because they exhibit the required dynamic range, linearity, and negligible image lag needed for these applications.
To understand the problems and causes of the waterfall effect and image-lag, please refer to FIGS. 8 and 9. FIG. 8 shows the CIS pixels and the readout circuit. Each pixel of a CIS consists of a npn phototransistor, its base-to-collector capacitance C.sub.pixel and a multiplexing switch connected as shown. In addition, for the entire line of sensors there is one common (shared) Reset switch and an analog output bus with its capacitance C.sub.out. FIG. 9 shows the timing of the clock input and the outputs of the n.sup.th and (n+1).sup.th stages of the scanning digital shift register which drive the multiplexing switches of the n.sup.th and (n+1).sup.th pixel. There is one clock cycle for each pixel, during which that pixel's multiplexing switch is closed in sequence from the first to the last pixels along the CIS array. During the first part of each clock cycle, called the sampling phase, the common Reset switch is open, so the selected pixel drives the analog output bus. During the second part of each clock cycle, called the Reset phase, the Reset switch is closed, and the output of the selected pixel is grounded. Every clock cycle during which a given pixel is not selected (its multiplexing switch is open) is referred to as that pixel's Integration phase.
Description of Operation
(1) Integration Phase: During a pixel's Integration phase, its multiplexing switch is open. Incident light induces charge carriers into the reverse-biased base-to-collector junction, so a current proportional to the light flows from collector to base. Charge accumulates on the base-to-collector capacitance C.sub.pixel increasing base voltage V.sub.base. At saturation, the base voltage will approach the voltage of the collector.
(2) Sampling Phase: When a pixel is selected for reading, its multiplexing switch is closed. Since the base-to-emitter junction is forward biased, a base current will flow from the base to the emitter. Consequently, a collector current multiplied by the current gain; h.sub.fe or Beta, of the phototransistor will flow into the analog output bus capacitance C.sub.out. The voltage on the bus V.sub.out rises and the voltage on the base V.sub.base falls slightly, until the voltage difference V.sub.be is reduced to the base-emitter junction threshold voltage V.sub.T, where the phototransistor turns off. Since this point is reached asymptotically slowly, usually V.sub.out is sampled before this time. As long as the sampling is done at a constant time delay after the multiplexing switch is closed, a valid image is obtained.
(3) Reset Phase: After V.sub.out has been sampled, the multiplexing switch is kept closed as the Reset switch is closed. This discharges the analog output bus capacitance C.sub.out directly, and also the base capacitance C.sub.pixel through the base-emitter junction of the phototransistor and through the multiplexing switch, until V.sub.base decreases to threshold V.sub.T.
Image-Lag Problem
Unlike the reset of C.sub.out, which is accomplished directly through the reset switch, the reset of C.sub.pixel is through the base-emitter junction and the multiplexing switch. This is a very slow process, and since the reset time is short, the result is an incomplete reset of the base capacitance C.sub.pixel. This is because the base-emitter on-resistance is dependent on the V.sub.be voltage. The resistance increases exponentially with decreasing V.sub.be and approaches infinity when V.sub.be approaches V.sub.T. The high on-resistance of the base-emitter junction makes the reset of C.sub.pixel asymptotically slow and results in carryover of signal charge into the next several scans.
FIG. 10 shows a plot of image lag or residual signal charge as a result of this incomplete reset of C.sub.pixel. The plot was obtained by using a 300 dpi CIS module which used the prior art npn phototransistors as detector elements. The module was exposed to a pulse light and then read out for several scans. The first scan represents the true signal while the remaining scans represent the residual image signal or image lag. The plot in FIG. 10 shows the residual image in the second, third, and forth scans in percentage relative to the output signal of the first scan. As can be seen from the plot, a residual image as high as 32%, 15%, and 9% is left behind in the second, third, and forth scans, respectively. This serious image-lag problem not only makes the scanned picture blurry, it also will cause non-linearity of photo-response.
Waterfall Effect
After imaging an extensive black area (no light) of the original document, and moving into a dark gray area, certain pixels continue to report "black" for a significant time, often several lines of scanning. The visual effect is black streaks downward from black areas of the document into dark gray areas below. The streaks resemble a waterfall of black ink, hence the name.
The cause of this waterfall effect is the sub-threshold leakage current of the base-emitter junction of the phototransistor during the reset periods in the dark. For ease of explanation, we have defined the threshold voltage V.sub.T as the voltage which when V.sub.be drops to this voltage value, the base-to-emitter forward-bias current cuts off. However in reality, the base-to-emitter forward-bias current never cuts off. Instead, the current will decrease exponentially with decreasing V.sub.be. For convenience, we say the forward-bias current cuts off when the current drops below a certain value. The small current flow through the base-emitter junction, when V.sub.be is below the V.sub.T value, is referred to as the sub-threshold leakage current of the base-emitter junction. During a long "dark" exposure time, this sub-threshold leakage current discharges C.sub.pixel so that V.sub.base &lt;V.sub.T (in other words, "blacker than black"). This condition is not cleared during the Reset phase, which depends on the assumption that V.sub.base &gt;V.sub.T in order to forward bias the base-emitter junction and return V.sub.base to V.sub.T. In other words, because the reset path is through the base-emitter diode, it can only pull V.sub.base down to V.sub.T, not up to it. Once V.sub.base &lt;V.sub.T, the only way C.sub.pixel can be recharged enough to forward-bias the base-emitter junction is by photoelectric current. In a white area, the photoelectric current is usually sufficient to restore V.sub.base &gt;V.sub.T and return to normal operation within a scan line time. However, if a black area is followed by a dark gray area, the photoelectric current charges C.sub.pixel very slowly, and several scan line times are necessary to forward bias the phototransistor again, thus resulting in a waterfall effect. Furthermore, this waterfall effect will also cause photo-response non-linearity in the low-light-level range.
Accordingly, it is an object of the present invention to provide a wide-dynamic-range contact image sensor (CIS) which permits use of phototransistor image sensors without the previous limitations of image lag, low-light-level thresholding or waterfall effect, and photo-response non-linearity.
It is a further object of the present invention to provide a CIS that has a simple device structure and sensitivity enhancement (or current gain) of a phototransistor detector and with performance improvements that can be used for color and gray-scale-sensitive scanning applications.
It is a further object of the present invention to provide a CIS that has a simple device structure and sensitivity enhancement (or current gain) of a phototransistor detector and can be operated with a speed much faster than previous phototransistor CIS arrays.
It is a further object of the present invention to provide a CIS that has smaller die size as compared with CIS with active photodiode detector and at the same time operates with performance that is adequate for both color and gray-scale-sensitive scanning applications.
It is a still further object of the present invention to provide a high-performance CIS that can be manufactured using a CMOS process, which is used in producing very high-volume CIS sensors for facsimile machine applications.